1. Field of the Invention
The invention relates to zeolite L-based reforming catalysts and their use to produce reformate having a lower content of C.sub.9 and C.sub.10 aromatic compounds.
2. Description of Related Art
Catalytic reforming is a major petroleum refining process used to raise the octane rating of naphthas (C.sub.6 to C.sub.11 hydrocarbons) for gasoline blending. Catalytic reforming is also a principal source of aromatic chemicals, i.e., benzene, toluene and xylenes, via conversion of paraffins and naphthenes to aromatics.
The principal reforming chemical reactions are dehydrogenation of cyclohexane to aromatics, dehydro-cyclization of paraffins to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, isomerization of normal paraffins to branched paraffins, dealkylation of alkylbenzenes, and hydrocracking of paraffins to light hydrocarbons. The hydrocracking of paraffins to light hydrocarbons is undesirable and should be minimized because light hydrocarbons have low value.
Catalysts commonly used in commercial reformers include a Group VIII metal, such as platinum, or platinum plus a second catalytic metal, such as rhenium or iridium, dispersed on an alumina substrate. Typically, chlorine is incorporated on the alumina to add acid functionality. Alumina-based reforming catalysts are suitable for aromatizing C.sub.8+ paraffins, but are less effective for aromatizing C.sub.6 to C.sub.8 paraffins because these catalysts hydrocrack more of the lighter paraffins to low value fuel gas than they convert to aromatics.
Conventional reforming catalysts are bifunctional, i.e., the catalysts enhance i) dehydrogenation and cyclization reactions on the catalytic metal sites; and ii) isomerization on separate strong acid sites in the catalyst. The undesirable hydrocracking reactions also occur on the acid sites.
Within the past few years reforming catalysts have been developed which have been discovered to be particularly effective for aromatizing the C.sub.6 to C.sub.8 paraffin components of naphtha. These catalysts are made using zeolite, rather than alumina, as the support for the catalytic metal. They are mono-functional and contain relatively few strong acid sites. Unlike conventional bifunctional catalysts, zeolite based catalysts accomplish dehydrogenation and cyclization reactions as well as isomerization on the dispersed metallic catalytic sites. Because these zeolite-based catalysts have few strong acid sites, undesirable hydrocracking reactions are repressed. Zeolites which are preferred for reforming catalysts are large pore zeolites i.e., zeolites with a 6 to 15 Angstrom pore diameter. Zeolite L is the most preferred support for reforming catalysts, particularly wherein the catalytically active metal is platinum. Examples of such catalysts are disclosed in U.S. Pat. Nos. 4,104,320 and 4,544,539.
Modified versions of these Group VIII metal-containing catalysts which also contain an ion-exchanged alkaline earth metal such as calcium, barium or strontium are disclosed in U.S. Pat. No. 4,435,283. This catalyst is disclosed to have a higher selectivity with respect to the dehydrocyclization of alkanes such as n-hexane into aromatics.
More recently, zeolite L crystallites having a cylindrical structure and shorter channel length have been developed which provide for improved run lengths, conversion and selectivity towards aromatics production when used as a catalyst support in a reforming process. These crystallites may be characterized as "coin" or "hockeypuck" shaped and have a relatively large diameter and short length. The "length" of a crystal is a measurement of the outer edge of the crystal perpendicular to the basal plane containing the diameter. The length is typically 0.1 to 0.6, preferably 0.1 to 0.3 microns and the diameter is generally 0.3 to 1.5 microns, preferably 0.4 to less than 1.0 micron. When the length/diameter ratio is 0.2 to 0.5, the crystal shape is termed "hockeypuck". When this ratio is less than 0.2, the shape is termed "coin".
These new zeolites are synthesized by hydrothermal treatment of a synthesis mixture containing water, a source of potassium, a source of Al.sub.2 O.sub.3, a source of SiO.sub.2 and up to about 0.1 wt %, based on the synthesis mixture, of a source of a divalent cation selected from the group consisting of magnesium, calcium, barium, manganese, chromium, cobalt, nickel and zinc. The divalent cation present in the synthesis mixture serves to reduce the size and regulate the shape of the resulting zeolite L crystallites and also suppresses the formation of unwanted impurities such as zeolite W.
These zeolites and platinum-loaded versions thereof used as reforming catalysts are disclosed in U.S. Pat. Nos. 5,486,498 and 5,491,119, the complete disclosures of which patents are incorporated herein by reference.
One of the primary goals in a naphtha reforming process is to achieve a reformate which contains a high content of aromatics because these chemicals are more valuable and contribute to higher octane values where the reformate is used in the gasoline product pool. Present requirements of the U.S. Clean Air Act and the physical and compositional limitations imposed by the Reformulated Gasoline (RFG) and U.S. EPA Complex Model regulations will result in limitations on gasoline boiling range, typically measured by minimum Reid Vapor Pressure (RVP) and T.sub.90 specification. What this means is that reformates containing a higher content of lighter aromatics, e.g., benzene, toluene and xylene (BTX) are more valuable for use in reformulated gasoline than reformates wherein the aromatics content also includes significant amounts of heavier C.sub.9 and C.sub.10 aromatics.
Accordingly, it is an object of the invention to provide a fresh or regenerated Group VIII metal-containing zeolite L catalysts which is highly selective to the production of aromatics containing lesser amounts of C.sub.9 and C.sub.10 aromatics.
Another object of the invention is to provide a process for reforming naphtha streams using this catalyst, as well as activated and regenerated versions thereof.
Still another object of the invention is to provide a process for further activating or regenerating this catalyst.